Beyond the Nucleophilic Role of Metal–Boryl Complexes in Borylation Reactions

Transition
metal–boryl complexes play an important role
as reactive intermediates in catalytic borylation processes and have
enjoyed tremendous exploration. Over the years, it has been well established
that a boryl ligand possesses strong nucleophilicity in copper­(I)–boryl
complexes. Thus, the role played by boron’s “empty”
p orbital has often been overlooked and d-electrons’ effect
from the metal is even more seldom mentioned, despite the fact that
they in theory should also have influence on the reactivity of a metal–boryl
complex. In this work, the reactivity of metal–boryl complexes
has been systematically studied with the aid of density functional
theory calculations. We have revealed that when the metal is more
electronegative, the nucleophilic feature of boryl is diminished and
will no longer be active, whereas when the boryl is not substituted
with π-donating groups, the “empty” p orbital
of boron will be active and can serve as the electrophilic site. In
addition, when the metal center changes from relatively inert d10 to d8, the d electrons can also play the role
of a nucleophile. With a much more comprehensive understanding of
metal–boryl complexes, this work will benefit the future catalyst
design related to transition-metal-catalyzed borylation reactions.